抄録
Absorption of light by rhodopsin causes a change in receptor potential as hyperpolarization for vertebrate rod outer segment and depolarization for invertebrate rhabdomal microvilli. Since rhodopsin is located in disk membranes which are separated from the external plasma membrane of the outer segment where the sodium conductance change occurrs, the concept of intermediary transmitter is proposed in order to interprete communication between the separated membranes. This concept apply equally to cephalopod microvilli in which rhodopsin is located in the plasma membranes. I will intend to review current knowledge of the molecular transmitter systems of vertebrate rod outer segment and cephalopod microvilli.
Isomerzation of retinal in rhodopsin by light induces conformational changes of opsin protein to produce intermediates. Metarhodopsin II, one of the intermediates in the photolysis of vertebrate rhodopsin, associates with Gprotein and information flows. Amplified information carrying mechanism take place in GTP/GDP exchange of G-protein. Phosphodiesterase is activated by G·GTP and hydorize cGMP. Activity of these enzymes is influenced by concentration of Ca++. Release of Ca++ from the disk membrane is influenced by these enzymes.
Although a lot of evidences seem sufficient to place cGMP and Ca++ in the transduction mechanism, the role of these substances are subject of controversy. Studies using cephalopod photoreceptor might give an insight into underestanding of the transduction mechansim because rhodopsin and ion channel located in the same plasma membranes.
